Just finished the book, and there are two thing I wish I'd known before I set out.
First, the commentaries tend to be on the brief side. The corresponding entries in "An A-Z of ELT" by the same author, on the other hand, are just right: they go into enough detail without being overly long and provide clarifying examples that really aid understanding. In short, if you are a "zero uncertainty" fan like me, I highly recommend supplementing "About Language" with "An A-Z of ELT".
Secondly, working through "About language" is more time-consuming and tiring than you'd expect. A chapter took me between 90 and 120 minutes. That includes doing the tasks, looking up relevant entries in "An A-Z of ELT", reading commentaries, taking notes on the margins and writing out definitions on separate cards. I had this optimistic estimate where I was going to do 2 chapters a day and finish the book in two or three weeks. Didn't happen. A chapter a day, four chapters a week turned out to be a more realistic pace. If you are planning to work through "About language" before DELTA, take it up 2-3 months in advance. That way you can alternate it with other reading, have breaks, and still make it.
Is it worth working through? Absolutely. Makes you feel real smart (also, prepares you for DELTA M1 Paper 1 Part 4).